21. Biotic Manipulations Involving Belowground Animals
aboveground biota are numerous, making the
above- and below ground systems interdependent.
Numerous organisms that forage and seek mates
aboveground spend a large majority of their time
belowground in dens and nests, utilize soil biota for
food, and/or have one stage of their life cycle in the
soil.
Recent reviews have shown below ground vertebrates and invertebrates have different functions,
some of them key to soil sustainability (Lavelle
1997; Brussaard et al. 1997; Hooper et aI., in press).
The invertebrates in soil can alter net primary productivity (NPP), soil structure, patterns of microbial activity, soil organic matter dynamics, and nutrient cycling (Mielke 1977; Grant et al. 1980; Cox
1984; Seastedt 1984; Anderson 1987; Huntly and
Inouye 1988; Brown and Gange 1990; Verhoef and
Brussaard 1990; Beare et al. 1992; Reichman et al.
1993; Heal et al. 1996; Lavelle 1997). For example,
depending on the system, earthworms can increase
water infiltration (Sharpley et al. 1979; Subler et
al. 1997), but, in some cases, accelerate nitrate
leaching (Subler et al. 1997). Soil invertebrates
also have potential utility as indicators of pollution
and other soil disturbance (Paoletti et al. 1991;
Freckman and Ettema 1993; Blair et al. 1996; Niles
and Freckman 1998). Vertebrates, whose contribution to functioning of soil systems is generally
underrated, change soil structure and chemistry
through wallowing and compaction (Lock 1972),
tunneling and mixing of soil organic matter (Kalisz
and Stone 1984; Anderson 1987), consuming roots
(Reichman and Smith 1985, 1991; Reichman and
Jarvis 1989), fracturing soil aggregates (Cortinas
and Seastedt 1996), dispersing symbiotic fungi,
microbes and invertebrates (Maser et al. 1978;
Johnson 1996; Herrera et al. 1997), generating egglaying sites (Huntly and Inouye 1988), and depositing feces and urine (Anderson 1987; Jaramillo and
Detling 1992).
Difficulties in experimentally evaluating the role
and influence of species in soil are magnified when
one considers the variation in the soil biota. These
difficulties include (1) size of the organisms (microscopic to 1.5 kg), (2) vast diversity of the species, many of which are undescribed (one group of
soil microarthropods, the orabatid mites, are estimated to have about 20% of the species described,
or 7000 described species in soils [Balogh and
Balogh 1992]), (3) life spans (minutes to years),
319
(4) the three-dimensional spatial scales over which
the animals exist (/lm to > 10 m depth), and (5) the
differences in their distribution over a landscape
(nested and clumped underneath plants vs. evenly
distributed). In addition, no single method exists for
sampling all organisms in soil, making quantification between microscopic invertebrate taxa (protozoa, nematodes, tardigrades, rotifers, mites, springtails) problematic (Anderson and Ingram 1993;
Hall 1996; Schinner et al. 1995; Blair et al. 1996;
Coleman et al. 1999; Wall and Virginia 1999).
In the soil only vertebrates and larger visible
macroinvertebrates (> 10 mm in length, i.e., termites, ants, earthworms, millipedes) are easily studied at the species level. Most ecosystem experiments of microfauna (0.5 to 200 /lm in length) and
mesofauna (200 /lm to 10 mm in length) have been
at the functional or trophic group level. In contrast
to aboveground systems, in which a high percentage of identified species with similar functions are
placed into trophic groups, soil ecologists have
trophic groups with a higher proportion of unidentified species with unknown life histories. Thus,
functional groups of soil biota are designated as
those organisms of similar morphology and function inferred from feeding habits of a few known
species within the group (Freckman 1982; Coleman
and Crossley 1996). This lack of knowledge associated with the species of functional groups affects
our estimates of nutrient cycling through the soil
food webs. Nevertheless, numerous studies at the
functional group level and collaboration on ecosystem experiments with taxonomists, have contributed significantly to our understanding of soil biota
and ecosystem functioning.
Soil biodiversity is structured into food webs that
appear to exert a strong influence on ecosystem
function (Moore and de Ruiter 1991; Heal et al.
1996; Swift and Anderson 1994). The majority of
ecosystem research has concentrated primarily on
microbial biomass and soil respiration, and neglected the contributions of soil protozoa, mycorrhizae, and invertebrates. These latter groups are
abundant in soils and have many direct and indirect
effects on ecosystem processes. They directly affect
ecosystems by affecting plant community composition, as herbivores, as processors of organic matter, and as regulators by preying on other animal
species. They indirectly affect processes as secondary consumers, regulating the rate of decomposi-
aboveground biota are numerous, making the
above- and below ground systems interdependent.
Numerous organisms that forage and seek mates
aboveground spend a large majority of their time
belowground in dens and nests, utilize soil biota for
food, and/or have one stage of their life cycle in the
soil.
Recent reviews have shown below ground vertebrates and invertebrates have different functions,
some of them key to soil sustainability (Lavelle
1997; Brussaard et al. 1997; Hooper et aI., in press).
The invertebrates in soil can alter net primary productivity (NPP), soil structure, patterns of microbial activity, soil organic matter dynamics, and nutrient cycling (Mielke 1977; Grant et al. 1980; Cox
1984; Seastedt 1984; Anderson 1987; Huntly and
Inouye 1988; Brown and Gange 1990; Verhoef and
Brussaard 1990; Beare et al. 1992; Reichman et al.
1993; Heal et al. 1996; Lavelle 1997). For example,
depending on the system, earthworms can increase
water infiltration (Sharpley et al. 1979; Subler et
al. 1997), but, in some cases, accelerate nitrate
leaching (Subler et al. 1997). Soil invertebrates
also have potential utility as indicators of pollution
and other soil disturbance (Paoletti et al. 1991;
Freckman and Ettema 1993; Blair et al. 1996; Niles
and Freckman 1998). Vertebrates, whose contribution to functioning of soil systems is generally
underrated, change soil structure and chemistry
through wallowing and compaction (Lock 1972),
tunneling and mixing of soil organic matter (Kalisz
and Stone 1984; Anderson 1987), consuming roots
(Reichman and Smith 1985, 1991; Reichman and
Jarvis 1989), fracturing soil aggregates (Cortinas
and Seastedt 1996), dispersing symbiotic fungi,
microbes and invertebrates (Maser et al. 1978;
Johnson 1996; Herrera et al. 1997), generating egglaying sites (Huntly and Inouye 1988), and depositing feces and urine (Anderson 1987; Jaramillo and
Detling 1992).
Difficulties in experimentally evaluating the role
and influence of species in soil are magnified when
one considers the variation in the soil biota. These
difficulties include (1) size of the organisms (microscopic to 1.5 kg), (2) vast diversity of the species, many of which are undescribed (one group of
soil microarthropods, the orabatid mites, are estimated to have about 20% of the species described,
or 7000 described species in soils [Balogh and
Balogh 1992]), (3) life spans (minutes to years),
319
(4) the three-dimensional spatial scales over which
the animals exist (/lm to > 10 m depth), and (5) the
differences in their distribution over a landscape
(nested and clumped underneath plants vs. evenly
distributed). In addition, no single method exists for
sampling all organisms in soil, making quantification between microscopic invertebrate taxa (protozoa, nematodes, tardigrades, rotifers, mites, springtails) problematic (Anderson and Ingram 1993;
Hall 1996; Schinner et al. 1995; Blair et al. 1996;
Coleman et al. 1999; Wall and Virginia 1999).
In the soil only vertebrates and larger visible
macroinvertebrates (> 10 mm in length, i.e., termites, ants, earthworms, millipedes) are easily studied at the species level. Most ecosystem experiments of microfauna (0.5 to 200 /lm in length) and
mesofauna (200 /lm to 10 mm in length) have been
at the functional or trophic group level. In contrast
to aboveground systems, in which a high percentage of identified species with similar functions are
placed into trophic groups, soil ecologists have
trophic groups with a higher proportion of unidentified species with unknown life histories. Thus,
functional groups of soil biota are designated as
those organisms of similar morphology and function inferred from feeding habits of a few known
species within the group (Freckman 1982; Coleman
and Crossley 1996). This lack of knowledge associated with the species of functional groups affects
our estimates of nutrient cycling through the soil
food webs. Nevertheless, numerous studies at the
functional group level and collaboration on ecosystem experiments with taxonomists, have contributed significantly to our understanding of soil biota
and ecosystem functioning.
Soil biodiversity is structured into food webs that
appear to exert a strong influence on ecosystem
function (Moore and de Ruiter 1991; Heal et al.
1996; Swift and Anderson 1994). The majority of
ecosystem research has concentrated primarily on
microbial biomass and soil respiration, and neglected the contributions of soil protozoa, mycorrhizae, and invertebrates. These latter groups are
abundant in soils and have many direct and indirect
effects on ecosystem processes. They directly affect
ecosystems by affecting plant community composition, as herbivores, as processors of organic matter, and as regulators by preying on other animal
species. They indirectly affect processes as secondary consumers, regulating the rate of decomposi-